1. Field of the Invention
The present invention relates to thermal transfer video printers, and more particularly, to improvement of temperature correction system of coloring density in a thermal transfer video printer of the type controlling coloring density by controlling the heating time period of the thermal head.
2. Description of the Prior Art
A color video printer is in practical use that prints out a picture of a reproduced motion picture as a color still picture, in reproducing an image shooted by a video camera or an image recorded by a video cassette recorder (referred to as VCR hereinafter). One such color video printer is the so-called sublimation type thermal transfer color video printer which uses sublimation dye as the ink for printing. By controlling a heating time period of each of a plurality of heating elements forming the thermal head according to the tone level of the corresponding pixel, the sublimation amount of sublimation dye of each pixel into a recording paper, i.e. the coloring density, is controlled. Such a sublimation type thermal transfer color video printer is disclosed in U.S. Pat. No. 4,691,211, for example.
FIG. 1 is a block diagram schematically showing the structure of a conventional sublimation type thermal transfer color video printer. Referring to FIG. 1, a video interface circuit 11 is provided with a video signal via an input terminal 9 from a video signal source not shown (for example, the image sensed output of a video camera or the reproduced output of a VCR). Video interface circuit 11 supplies the applied video signal directly to a monitor TV unit not shown via an output terminal 10. Video interface circuit 11 also derives luminance data and color difference data forming the video signal to provide the same to an image processing unit 12.
Image processing unit 12 is formed of an image memory and the like to temporarily hold data relating to luminance and color difference of one picture provided from video interface circuit 11. These data are converted into print signals of three primary colors of cyan, magenta and yellow, and then subjected to a predetermined image process such as edge enhancement of the image. The print signals of three primary colors provided from image processing unit 12 are applied to a microcomputer 8 functioning as a system controller. The output of microcomputer 8 is applied to a thermal head control unit 1 and a mechanism driving unit 4 that will be explained later.
Thermal head 2 is constituted as a unit including a heating element 3 and periphery circuits not shown. The operation of thermal head 2 is controlled by thermal head control unit 1 according to the signal from microcomputer 8. Heating element 3 is formed of 480 small heating elements arranged in a longitudinal direction on a straight line corresponding to one horizontal line, as shown in the aforementioned U.S. Pat. No. 4,691,211. The heating operation thereof will be explained in detail afterwards. Thermal head 2 is provided with a thermistor 13 to detect the temperature of thermal head 2. This detected signal is provided to microcomputer 8.
A recording paper 5 is wound around a platen roller P. Platen roller P is rotated by a platen roller driving motor M1 so that recording paper 5 is forwarded a distance of circumference corresponding to the width of 1 horizontal line for every printing corresponding to one horizontal line of an image.
An ink sheet containing the aforementioned sublimation dye is wound around a take-up reel 6a and a supply reel 6b constituting an ink sheet cassette. Ink sheet 7 is formed of three types of ink sheets of yellow, magenta and cyan arranged along the longitudinal direction. Take-up reel 6a is rotated by an ink sheet winding motor M2 so that the ink sheet of one of the primary colors, for example yellow, is rolled up by takeup reel 6a upon termination of printing one picture of yellow, followed by the ink sheet of the next color, for example, magenta rolled up by take-up reel 6a upon termination of printing one picture of magenta, and finally the cyan ink sheet rolled up by take-up reel 6a upon termination of printing of one picture of cyan.
Platen roller driving motor M1 rotates platen roller P so that the print start position on the recording paper (the position corresponding to the first horizontal line) comes to the position corresponding to heating element 3 after each printing of one of the three primary colors. Mechanism driving unit 4 controls the rotation of motors M1 and M2 according to a signal from microcomputer 8 in the above described manner.
In order to control the coloring density of the recording paper according to the tone level of each pixel in the reproduced image in a thermal transfer color video printer of the above described structure, the degree of absorption of sublimation dye of each color included in ink sheet 7 into the recording paper is varied for each pixel, by controlling the heating time period of each heating element forming thermal head 2, i.e. by controlling the duration of pulse applied to each heating element. The printing operation of such a conventional sublimation type thermal transfer printer will be explained hereinafter.
FIG. 2 is a graph showing the relation (coloring characteristic) between the heating (energized) time period of one heating element 3 forming thermal head 2 (the abscissa) and the coloring density in a recording paper that is the thermal transfer medium (the ordinate). Referring to the ordinate of FIG. 2, density D=0 indicates the state where incident light upon the recording paper is reflected by 100% (D=-log.sub.10 (100/100)); density D=1 indicates the state where the incident light is reflected by 10% (D=-log.sub.10 (10/100)); and D=2 indicates the state where incident light is reflected by only 1% (D=-log.sub.10 (1/100)). The recording paper itself is not absolutely white and slightly contains density itself (for example, D=0.05). In the following description of the conventional example, density 0.05 to 2.0 is divided into 128 tone levels, whereby density 0.05 of the recording paper itself is defined as the 0th tone step, and density 2 is defined as the 127th tone step.
Referring to heating time period T of the abscissa of FIG. 2, the heating time period required by the thermal head right before the recording paper colors is T.sub.0. The heating time periods of the thermal head required for carrying out printing of respective tone levels after coloring are respectively .DELTA.T.sub.1, .DELTA.T.sub.2, .DELTA.T.sub.3, . . . .DELTA.T.sub.126 and .DELTA.T.sub.127. Although these heating time periods vary according to various factors that will be explained afterwards, T.sub.0 is typically approximately 3 m seconds, and heating time period .DELTA.T.sub.i of each tone level is typically approximately 100 .mu. seconds to 200 .mu. seconds.
In the case of actually printing to a recording paper, heating element 3 of thermal head 2 must be heated for the time period of (T.sub.0 +.DELTA.T.sub.1) in printing the coloring density of the first tone step. In this case, thermal head control unit 1 functions to apply a pulse having a duration of (T.sub.0 +.DELTA.T.sub.1) to thermal head 2 to energize heating element 3 during this time period. In the case of printing the coloring density of the second tone step, heating element 3 of thermal head 2 must be heated for the time period of (T.sub.0 +.DELTA.T.sub.1 +.DELTA.T.sub.2). In this case, thermal head control unit 1 functions to apply a pulse having a duration of (T.sub.0 +.DELTA.T.sub.1 +.DELTA.T.sub.2) to thermal head 2 to energize heating element 3 during this time period. Similarly, in the case of printing the coloring density of the i-th tone step, heating element 3 of thermal head 2 must be heated for a time period of (T.sub.0 +.DELTA.T.sub.1 +.DELTA.T.sub.2 + . . . +.DELTA.T.sub.i). In this case, thermal head control unit 1 functions to apply a pulse having a duration of (T.sub.0 +.DELTA.T.sub.1 +.DELTA.T.sub.2 + . . . +.DELTA.T.sub.i) to thermal head 2 to energize heating element 3 during this time period. Energization of heating element 3 according to the pulse provided from thermal head control unit 1 is carried out by a power supply not shown and a drive circuit provided for each heating element included in thermal head 2.
Thermal head control unit 1 comprises a memory (not shown) storing data for determining the above described various heating time periods of T.sub.0, .DELTA.T.sub.1, .DELTA.T.sub.2, . . . , .DELTA.T.sub.127. The contents of this memory is shown in FIG. 3. In the above described conventional embodiment, the heating time period of heating element 3, i.e. the duration of heating pulse provided from thermal head control unit 1, is determined by counting the number of clock pulses having a predetermined period. In other words, the duration of a pulse for energization is a time period having a length that is an integer multiple of the clock period. The contents of the memory of FIG. 3 comprises data L.sub.0, .DELTA.L.sub.1, .DELTA.L.sub.2, . . . , .DELTA.L.sub.127 indicating the number of clock pulses to be counted for determining the heating time period for each tone level. The contents of this memory is referred to as the look up table or the tone table hereinafter. The relation between each data of the tone table and the actual heating time period in printing each tone level is as follows.
Assuming that the predetermined period of the clock pulse is CK, the following relationships are established between L.sub.0 and T.sub.0 and between .DELTA.L.sub.i and .DELTA.T.sub.i. EQU T.sub.0 =L.sub.0 .times.CK EQU .DELTA.T.sub.i =.DELTA.L.sub.i .times.CK (i=1 to 127)
Therefore, the heating time period in printing the first tone step is: EQU T.sub.0 +.DELTA.T.sub.1 =L.sub.0 .times.CK+.DELTA.L.sub.1 .times.CK
and the heating period in printing the i-th tone step is: EQU T.sub.0 +.DELTA.T.sub.1 + . . . +.DELTA.T.sub.i =L.sub.0 .times.CK+.DELTA.L.sub.1 .times.CK+ . . . +.DELTA.L.sub.i .times.CK
According to the signal provided from microcomputer 8, thermal head control unit 1 obtains data corresponding to the tone level for each pixel on each horizontal line from the tone table of FIG. 3 to count the clock pulses according to this data, whereby a heating pulse having the required duration is produced and provided to thermal head 2. With the above described operation, printing to a recording paper is achieved with coloring density corresponding to the tone level of the image to be printed out. The contents of the memory of FIG. 3 requires 1 byte as the storage capacity for each tone level. This means that if all the data of the 128 tone steps are to be stored, a storage capacity of 128 bytes is necessary for the entire memory.
In the above described conventional sublimation type thermal transfer color video printer, density of printing varies depending on the change in the environment temperature and rise in temperature of the thermal head itself. FIG. 4 shows the change in coloring characteristic of the recording paper according to change in temperature of the thermal head (temperature drift). Referring to FIG. 4, curve t.sub.0 indicates the coloring characteristic curve when the environment temperature (or the temperature of the thermal head) is t.sub.0, and curve t.sub.1 shows the coloring characteristic curve when the environment temperature (or the temperature of the thermal head) is t.sub.1 which is lower than t.sub.0. It can be seen from FIG. 4 that difference in environment temperature causes different heating time period T.sub.0 required for right before the coloring of the recording paper and heating time period .DELTA.T.sub.i required for printing each tone level. A longer heating time period is necessary to carry out printing of the same density as the environment temperature is lower.
It has been confirmed experimentally that a change in coloring density which can be visually recognized by the human eye occurs if there is a temperature change of 1.degree. C. or more. The temperature of the thermal head rises approximately by 3.degree. C. to 4.degree. C. for every one picture printing of each color of yellow, magenta and cyan. Correction in heating time period according to the environment temperature is necessary to correct the change in coloring density due to such change in temperature. For this purpose, a conventional sublimation type thermal transfer color video printer has a thermistor 13 provided in thermal head 2, as shown in FIG. 1. The temperature of the thermal head is estimated according to the detected signal provided from thermistor 13 to correct temperature of the heating time period of heating element 3.
More specifically, the relation between data L.sub.0, .DELTA.L.sub.i and heating time period T.sub.0, .DELTA.T.sub.i under the environment temperature of t.sub.0 is defined as follows: EQU L.sub.0 (t.sub.0)=T.sub.0 (t.sub.0)/CK EQU .DELTA.L.sub.i (t.sub.0)=.DELTA.T.sub.i (t.sub.0)/CK (i=1 to 127)
The relation under the environment temperature of t.sub.1 is as follows: EQU L.sub.0 (t.sub.1)=T.sub.0 (t.sub.1)/CK EQU .DELTA.L.sub.i (t.sub.1)=.DELTA.T.sub.i (t.sub.1)/CK
Data L.sub.0 and .DELTA.L.sub.i are calculated in advance according to the environment temperature to prepare a tone table for each environment temperature to be stored in a memory (ROM). According to the temperature of the thermal head detected by the thermistor, a tone table of the corresponding environment temperature is selected, whereby data L.sub.0 and L.sub.i in that table are transferred to the memory (RAM). By determining the heating time period according to the selected data, printing can be carried out at constant tone levels irrespective of the environment temperature.
It is necessary to switch the tone table for approximately every 1.degree. C. in order to prevent the change in density from being perceived visually by a human eye. Since the environment temperature that the thermal head operates is typically in the range of 5.degree. C. to 70.degree. C., it is necessary to prepare a total of 66 tone tables and store the same in advance in the ROM of thermal head control unit 1. The storage of 66 tone tables requires a ROM having a memory capacity of 128.times.66=8448 bytes since the required storage capacity for one tone table is 128 bytes, as mentioned before. This storage capacity is required for each color of cyan, magenta and yellow to result in a total of 8448.times.3=25, 344 bytes of storage capacity for the three colors. There was a problem that this total storage capacity is too large to implement a thermal head control unit 1 including this memory as a microcomputer of one chip.
Instead of providing the above-described ROM of a large capacity to calculate and store in advance tone tables corresponding to various environment temperatures, a method of obtaining data of a tone table by calculation is considered in accordance with the change in environment temperature during printing. The coloring density characteristic shown in FIG. 4 can be approximated by a predetermined function with environment temperature t as a variable. Therefore, the stored contents of the RAM can be modified by newly calculating the contents of the tone table according to the temperature of the thermal head detected by the thermistor for every 1 cycle of heating, i.e. for every printing of 1 horizontal line. This method has a disadvantage that a microcomputer having a very high operation speed is necessary since some time period is required for the calculation of the tone table every time one horizontal line is printed.